Social structure and spatial distribution of bottlenose dolphins (Tursiops truncatus) along the Croatian Adriatic coast

Abstract

Previous studies suggest that the common bottlenose dolphin population in the Croatian Adriatic Sea is structured into distinct local communities. However, their home ranges and levels of interaction remain unknown. Dedicated boat‐based surveys were conducted seasonally, in three adjacent study sites from 2013 to 2017. Photo‐identification data were used to identify distinct local communities by relating the social structure descriptors to the home ranges of individual animals, and to determine the community home ranges and identify the levels of interactions between them. The overlaps between the home ranges of individuals (50 and 95% kernel density estimators [KDE]) were positively correlated to association strengths between them. Three distinct communities were identified by the hierarchical cluster analysis. The individuals' 95% KDE home range overlaps and association strengths were significantly higher within the three communities than between them, indicating both social and spatial segregation. The 95% KDE home ranges of the communities showed very low to no overlap. The individuals ranged mainly centrally within the home range of their respective community. The probabilities of movement, expressed as lagged identification rates, were significantly higher within than between community home ranges. However, 37 out of 1024 identified individuals were observed in more than one community home range, indicating that the communities are not isolated. Four out of the six Sites of Community Importance currently in place within the Croatian Adriatic Sea lie within the home ranges of the three local bottlenose dolphin communities. Once these sites are declared as Special Areas of Conservation, the management scheme will need to accommodate for differences in the spatial and social ecology of these communities, and environmental and anthropogenic factors acting upon them. The results of this work provide baseline information for such tailored management.

Bottlenose dolphins (Tursiops truncatus) in North Dalmatia, Croatia: Occurrence and demographic parameters

“…As found by Pleslić et al (2019), the bottlenose dolphin community in North Dalmatia is in limited contact with neighboring communities along shore but has a notable influx of transients from the offshore part of the Adriatic Sea. Therefore, to account for these movements, natality and mortality, Pollock's Robust Design (Pollock, 1982) was used to estimate abundance for each summer season, and apparent survival and temporary emigration rates between consecutive summer seasons.…”

“…Furthermore, secondary occasions were carefully selected to maintain consistency in terms of area coverage. Given that the local resident community was not closed to migrations (Pleslić et al, 2019), the differences in annual abundance estimates likely reflect interannual variations in ranging patterns, rather than inconsistent effort or actual changes in abundance. This is further supported by the stability of group encounter rates across years.…”

“…As such, it provides baseline information in support of conservation measures and benchmark for future monitoring, particularly to detect any trend in abundance and to further explore reproductive capacity. The two Sites of Community Importance (SCI) for bottlenose dolphins currently in place under the Natura 2000 scheme cover 44% of the North Dalmatian community home range (Figure 1; Pleslić et al, 2019). To comply with the European Union Habitat Directive which requires ensuring favorable ecological conditions for survival of the local bottlenose dolphin community (Council Directive 92/43/EEC; Council of the European Communities), conservation measures within these SCIs will have to target those anthropogenic factors in the area that are recognized as having a detrimental effect on bottlenose dolphins: prey depletion, fisheries interactions, and marine traffic through physical and acoustic disturbance (Bearzi et al, 2008b; Bejder et al, 2006; Rako et al, 2013b).…”

“…The home range of the local community in north Dalmatia (see Pleslić et al, 2019) was used as the boundaries of the study area, except for a part west of the island of Dugi Otok where research effort was significantly lower (Figure 1). The study area encompasses approximately 2,500 km 2 of the North Dalmatian archipelago, from the island of Sestrunj in the north to the island of Žirje in the south (Figure 1).…”

“…A recent study by Pleslić et al (2019) found another distinct bottlenose dolphin community of at least 49 adult individuals in North Dalmatian waters. Based on estimated home ranges and social structure, this community is spatially and socially distinct from those in the Cres‐Lošinj and Vis areas.…”